In color television systems according to PAL or NTSC standards, a chrominance component in the form of a quadrature modulated color subcarrier, a luminance component, a blanking component and a synchronizing component are transmitted as a composite television signal. The luminance and chrominance components are frequency interleaved. At a receiver, the luminance and chrominance components are separated before being further processed prior to being applied to image reproduction apparatus. Such separation is also necessary in a video recorder system where the chrominance signal is subjected to special processing as is known.
It is known to pass the signal through a band pass filter to a line delay, and to produce a comb filtered chrominance signal by combining the signals from input and output of the line delay. A separated luminance component is provided by subtracting the comb filtered chrominance signal from the composite television signal. Such a comb filter produces an averaging over several image lines because signals of successive lines are combined. Thus, for example, in a signal with high frequency signal jumps (i.e., rapid transitions) or color transitions in the vertical direction, the comb filtered output signal may exhibit unwanted signal disturbances and artifacts. That is, the unwanted artifacts may appear in the comb filtered output signal when successive image lines contain different (uncorrelated) information such as produced by a color (phase) change or a rapid (amplitude) transition of a high frequency signal. This effect is indicated by the following table which illustrates the disturbed condition of several image lines after comb filtering.
______________________________________ line direct signal after comb filter ______________________________________ 2 . . . . . . . . .-.-.-.-.-.-.-.-.- (disturbed) 3 . . . . . . . . .-.-.-.-.-.-.-.-.- (disturbed) 4 . . . . . . . . . . . . . . . . 5 . . . . . . . . . . . . . . . . 6 .-.-.-.-.-.-.-.-.- (disturbed) 7 (disturbed) 8 ______________________________________
Certain improved transmission systems such as S-VHS video recorders and high definition television systems require a comb filter compatible with increased demands of such systems.
It is an object of the invention to develop a comb filter of the type described wherein signal disturbances and artifacts are significantly reduced or avoided while maintaining good signal separation.